This application relies for priority upon Korean Patent Application No. 2001-07914, filed on Feb. 16, 2001, the contents of which are herein incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to a flash memory device and, more particularly, to a method of programming a flash memory device with a NAND-type cell array.
2. Background of the Invention
Flash memory devices are block or sector-erasable memory devices, and data readable/writable non-volatile memory devices. As one of the flash memory devices, a flash memory device with a NAND-type flash memory cell array (hereinafter referred to as xe2x80x9cNAND flashxe2x80x9d) benefits from high integration, in which memory cell strings composed of memory cells are serially connected to a bitline. A NAND flash having such a configuration is disclosed in the paper ISSCC 1995, pp. 128-129, published in 1995.
FIG. 1 is a diagram of a memory cell array as taught in the ISSCC 1995 paper. A plurality of memory cell strings are each connected to a plurality of bitlines. FIG. 1 is transversed by wordlines WL1-WL16.
A unit cell string is associated with a single bitline intersecting a number of wordlines. For example, a unit cell string is composed of a string select transistor SST connected to bitline BL1, a ground select transistor GST connected to a common source line CSL, and a plurality of memory cells (or cell transistors) M11-M161 serially coupled between the select transistors SST and GST. Gates of the memory cells M11-M161 are each coupled to wordlines WL1-WL16.
Each memory cell is composed of a floating gate isolated from a channel region by a tunnel oxide layer (having a thickness of about 100 xc3x85) interposed therebetween, and a flash EEPROM (electrically erasable and programmable read only memory) having a control gate stacked over the floating gate with an interlayer insulating layer interposed therebetween.
Memory cells are now compared, to illustrate a difference in their treatment.
FIG. 2A is an equivalent circuit diagram of a selected memory cell in programming. Referring now to FIG. 2A, if a voltage Vcg is applied to a wordline coupled to a control gate, a voltage Vfg applied to a floating gate is determined by a coupling ratio of a capacitance Ci between the floating gate and the control gate to a capacitance Ct between the floating gate and the channel region.
Vfg=[Ct/(Ct+Ci)]xc3x97Vcgxe2x80x83xe2x80x83[FIG. 2A, selected cell]
FIG. 2B is an equivalent circuit diagram of an unselected memory cell in programming. Referring now to FIG. 2B, if a voltage Vcg is applied to an unselected cell, a voltage Vfg applied to a floating gate is determined by:
Vfg=[(Ci+Cch)/(2Ci+Cch)]xc3x97Vcgxe2x80x83xe2x80x83[FIG. 2B, unselected cell]
It will be noted that Vfg is different for selected and unselected cells.
FIG. 3A is a table showing applied voltages used for program, erase, and read operations in the NAND flash of FIG. 1. FIG. 3B shows the sequence of applying voltages in the prior art.
Referring to FIGS. 1, 3A, 3B, it is assumed that M21 is a selected memory cell. Then 0V is applied to a selected bitline (e.g., BL1) and the ground select line GSL. A power supply voltage Vcc of about 3V is applied to a unselected bitline BL2 and to a string select line SSL. A program voltage Vpgm of about 20V is applied to the selected wordline (in this case, WL2). And, a pass voltage Vpass of about 12V is applied to the unselected wordlines WL1, WL3-WL16.
The applied 0V is applied to a channel of the selected memory cell M21 through the string select transistor SST1 and the memory cell M1. A voltage difference between the floating gate and the channel results in F-N (Fowler-Nordheim) tunneling. Electrons are then moved from the channel region to the floating gate, enabling a threshold voltage to increase in 1V.
An erase operation is carried out with a wordline unit (or page unit; memory cells coupled to one wordline composes one page). Generally, memory cells coupled to all wordlines in a unit cell string are defined with a block unit (or sector unit), which is used as an elementary unit of the erase.
If 0V is applied to a selected wordline and an erase voltage Vers of about 24V is applied to a bulk area (separated with a block or sector unit), an inverse electric field to a program operation is formed, which moves electrons from the floating gate to the bulk area. As a result, the threshold voltage of the memory cell is changed to xe2x88x923V.
The read operation utilizes the fact that a threshold voltage is changed according to a state of a selected memory cell (i.e., whether the selected memory cell is programmed or erased). When 0V is applied to a selected bitline, unselected wordlines, a string select line SSL, and a ground select line GSL, and 0V is applied to a selected wordline WL2, if a programmed cell is a programmed cell (off-cell), there is no current flowing through the memory cell. And, if the programmed cell is an erased cell (on-cell), a current flowing through the memory cell is generated. A voltage of a bitline reacting to whether a current flows or not according to the memory cell state is sensed to carry out a read operation.
An undesirable programming disturbance phenomenon occurs in a string program inhibit cell or a page program inhibit cell while programming a program cell. In this case, the xe2x80x9cprogram cellxe2x80x9d means a memory cell coupled to a selected bitline and a selected wordline; the xe2x80x9cstring program inhibit cellxe2x80x9d means an unselected memory cell belonging to the same string cell as a memory cell (or selected memory cell) coupled to a selected bitline and an unselected wordline; and the xe2x80x9cpage inhibit cellxe2x80x9d means an unselected memory cell coupled to an unselected bitline and a selected wordline or an unselected memory cell belonging to the same page as a selected memory cell.
The programming disturbance phenomenon is that, although such unselected memory cells should not be programmed while programming a selected wordline, they do not retain initial states; in other words, they also become programmed. This is because such unselected memory cells are connected to the selected wordline or a selected bitline from a standpoint of the otherwise desirable circuit configuration of FIG. 1.
A characteristic of the programming disturbance phenomenon varies in accordance with intrinsic program or erase states of the program inhibit cells.
To prevent the phenomenon, after obtaining a set width of the pass voltage Vpass based upon the states of the program inhibit cells, a pass voltage is set and applied to a program operation in consideration of a worst case.
Setting the pass voltage changes a channel voltage, causing F-N tunneling contributing to the programming disturbance, because there is a voltage difference of 3V between threshold voltages of a programmed memory cell and an erased memory cell. That is, a channel voltage of the program inhibit cell is set to prevent F-N tunneling.
For example, a voltage of a floating gate is boosted by a program voltage Vpgm in a program inhibit cell M22 coupled to a selected wordline WL2 and an unselected bitline BL2. On the other hand, a channel region of a memory cell is isolated from an external voltage to be floated by 0V applied to GSL and Vcc applied to SSL. If Vpass and Vpgm are applied then, an overall capacitance (Cg) in a gate formed by the foregoing Ci and Ct (control gate+floating gate) can extract an equation xe2x80x9c1/Cgate=1/Ci+1/Ctxe2x80x9d. If the channel capacitance (Cchxe2x80x2) meets the following equation xe2x80x9cCchxe2x80x2=Cch+Cjnc (junction capacitance at a source and a drain)xe2x80x9d, a voltage (Vch) induced to a channel of a page program inhibit cell M22 is represented hereinbelow.
Vch=Vpassxc3x97(Cg+Cchxe2x80x2)xc3x97(WL numberxe2x88x921)+(Cg/(Cg+Cchxe2x80x2))xc3x971
In the same page program inhibit cell as M22, Vch is formed to lower a strength of an electric field between a floating gate and a channel region. Accordingly, M22 can suppress an unwanted program formed by a program voltage Vpgm applied to a control gate. In case of a program inhibit cell such as M22 coupled between a selected wordline WL2 and an unselected bitline BL2, the higher Vpass is, the greater an effect to suppress a programming disturbance (unwanted program) is.
On the other hand, in case of a string program inhibit cell M11 coupled to an unselected wordline WL1 and a selected bitline BL1, electrons are implanted into the floating gate by Vpass applied to WL1 and 0V applied to a bulk area. This results in the formation of an unwanted program (especially, referred to as xe2x80x9cpass voltage disturbancexe2x80x9d). Therefore, Vpass is applied with a lower level than Vpgm.
Referring to FIG. 3C, a graph illustrates a relationship between a position of a program inhibit cell and a pass voltage. It should be remembered that the cell position is associated with a threshold voltage of the cell.
As will be seen below, the problem in the prior art is that the program disturb curves of unselected memory cells are not identical to each other. Indeed, for less than 7V, the graphs diverge. That is because where a conventional memory device is being programmed, a precharged channel voltage of an unselected memory cell having an erased state is different from that of having a programmed state.
More particularly, if Vpass is low (about 7V and lower), a programming prevention effect is good to a string program inhibit cell, while an unselected memory cell corresponding to the page program inhibit cell is adversely programmed by the program disturbance to boost its threshold voltage. On the other hand, if Vpass is high (about 14V and higher), a programming prevention effect is good to a gate program inhibit cell, while an unselected memory cell corresponding to a string program inhibit cell is adversely programmed by the program disturbance to boost its threshold voltage. Therefore, it is important to suitably set a range of the pass voltage Vpass in consideration of the foregoing relationship. The pass voltage range is called xe2x80x9cpass voltage windowxe2x80x9d. The wider the pass voltage window is, the greater an operation margin of the pass voltage.
In case of the program disturbance, a channel boosting voltage of a memory cell M12, coupled between WL2 to which Vpgm is applied and WL1 situated between bitlines, is variable to a state of the memory cell M12, as shown in FIG. 3C. This means that when Vpgm is applied to a gate of M22 and Vpass is applied to a gate of M12, a voltage formed at a channel of M22 is affected by a state of M12 because M12 and M22 are adjacent to each other.
That is, since a threshold voltage of M22 increases by 1V when M12 is put at a programmed state, a channel is not turned on and a channel voltage is not boosted until a voltage of WL2 is at least 1V and higher. When M12 is put at an erased state, a threshold voltage of M22 is xe2x88x923V and the channel is turned on, although a voltage of WL2 is 0V. As the voltage of WL2 is boosted to be Vpgm after Vcc applied to BL2 is partially applied to the channel of M22 (in this case, a channel voltage boosting amount is denoted as xe2x80x9caVxe2x80x9d), a high-level channel voltage can be obtained. Based upon whether M12 was in a program state or in an erase state, the boosted voltage of M12 is represented hereinbelow.
Vchp=[(Vpassxe2x88x921V)xc3x97bxc3x97(WL numberxe2x88x921)+(Vpgmxe2x88x921V)xc3x97b]/WL number (M12 was in a program state)
Vche=a+[(Vpassxc3x97bxc3x97(WL numberxe2x88x921)+Vpgmxc3x97b]/WL number (M12 was in an erase state)
*b=Cg/(Cg+Cch)
When M12 is placed in the erase state, a channel voltage Vche of M22 is higher than Vchp (a channel voltage of M33 when M12 is placed in a program state, i.e., Vche greater than Vchp. If M12 is programmed, the programming disturbance becomes worse. If an unselected memory cell located between a page program inhibit cell and its bitline is programmed, it is hard to obtain a channel voltage enough to prevent unwanted F-N tunneling. In other words, this means that the programming disturbance more easily occurs than in the erase state. Accordingly, a pass voltage Vpass used in the program operation is determined to a suitable one between values in the program state and in the erase state.
However, if a program progresses using the determined pass voltage Vpass, a channel voltage of the erased unselected memory cell is excessively higher than that of the programmed unselected memory cell. A great amount of current is then generated by function breakdown. And hot carriers, additionally formed, increase a trap density at a source and drain junction and a tunnel oxide layer. This results in early deterioration of the memory cell. Furthermore, the program operation is repeated to create a junction breakdown stress, which breaks down the junction to cause errors of the memory cell. Thus, a junction breakdown voltage of the memory cell must be higher than the boosted channel voltage.
Punch-through and isolation characteristics of the memory cell must be maintained, even when integration is high. To accomplish this in the prior art, impurities of an opposite conductive type to source and drain regions are heavily implanted into an isolation area between bulk and channel areas and the memory cell. Unfortunately, such a process cannot be realized substantially.
The present invention overcomes the limitations of the prior art. The invention provides methods of realizing a reliable program inhibit function in a NAND flash memory device.
A programming method according to the invention comprises applying a first voltage to one or more of the unselected wordlines. Then a predetermined bitline voltage is applied to an unselected one of the bitlines. And in a third step, a second voltage is applied to the unselected wordlines, and a third voltage is applied to a selected wordline out of the wordlines.
The invention further succeeds in forming a constant channel voltage, which depends on states of a memory cell in a program operation of a NAND flash.
The invention further succeeds in preventing excessive boosting of a channel voltage of an unselected memory cell which is put in an erase state, even though a program operation is repeated in an NAND flash.